"The insured" as used in paragraphs (1), (2), (3), and (4) shall mean only that insured under the policy against whom the particular claim is made or suit brought. "An insured" as used in paragraphs (5) and (6) shall mean any insured under the policy including those persons who would have otherwise been included within the policy's definition of an insured but, by agreement, are subject to the limitations of paragraph (1) of subdivision (d).
An agreement made by the insurer and any named insured more than 60 days following the inception of the policy excluding a designated person by name shall be effective from the date of the agreement and shall, with the signature of a named insured, be conclusive evidence of the validity of the agreement.
That agreement shall remain in force as long as the policy remains in force, and shall apply to any continuation, renewal, or replacement of the policy by the named insured, or reinstatement of the policy within 30 days of any lapse thereof.
Ca. Ins. Code § 11580.1